A reactor is used while an alternating current is superimposed on a direct current. If the fundamental wave current component is regarded approximately as a direct current, a phenomenon in which a high-frequency noise current is superimposed while a motor or a generator is operating can be regarded as a situation where an alternating current is superimposed on a direct current. Therefore, when electrical equipment such as a reactor or a motor is designed, inductance related to an alternating current superimposed on a direct current, or inductance related to an AC magnetic field superimposed on a DC magnetic field, is required to be calculated with a high degree of accuracy at high speed.
As methods for calculating inductance related to an alternating current superimposed on a direct current, various methods have been suggested. For example, by the technique disclosed in PTL 1, the operating point for a predetermined direct current is determined by a magnetic field analysis simulator, and the result is set as the initial value. Meanwhile, incremental magnetic permeability is determined from a table indicating a relationship between a predetermined magnetic flux density and incremental magnetic permeability, and an alternating current analysis is carried out to obtain an inductance value. By this method, however, a transient analysis (an analysis method by which a magnetic field is analyzed at each moment while the clock is set forward little by little, and this step is repeated a number of times) is carried out in an alternating current analysis after the operating point is determined. Therefore, a long analysis time is required.
By the techniques disclosed in PTL 2 and PTL 3, a transient analysis is not carried out, and high speed is achieved by calculating inductance through a single static magnetic field analysis. However, accuracy of the calculation is poor, since the eddy current flowing in the iron core is not taken into account. In view of this, PTL 4 discloses a technique by which incremental magnetic permeability having the influence of the eddy current taken into account is actually measured by using a ring-shaped sample, and a magnetic field analysis is carried out by using the incremental magnetic permeability, so as to take into account the influence of the eddy current in a single static magnetic field analysis. However, the process is complicated, and calculation accuracy is limited.
Therefore, programs have been suggested recently, such as a commercially-available magnetic field analysis program for carrying out an alternating current analysis after determination of an operating point through a higher-speed frequency response analysis (an analysis method for analyzing steady states in complex-number domains on the assumption that a magnetic flux density varies sinusoidally with time when a sinusoidal current is input), instead of a transient analysis (see NPTL 1). However, accurate modeling of a relationship between the initial magnetization curve and a minor loop is hardly realized. Specifically, by the magnetic field analysis program disclosed in NPTL 1, modeling is performed so that the magnetic field becomes larger or smaller from a point on the initial magnetization curve, though a minor loop should be formed so that the magnetic field becomes smaller from a point on the initial magnetization curve. As a result, there remains a program that a solution different from that in an actual phenomenon is found, with the upper endpoint of the minor loops not existing on the initial magnetization curve.